1. Field of the Invention
The present invention relates to an ultrasonic motor having a stator vibrated by a piezoelectric element and a rotor rotating slidably in contact with the stator.
2. Description of Related Art
An example of a conventional ultrasonic motor driven by standing-wave is shown in FIG. 19. The ultrasonic motor is composed of a stator 51 and a rotor 50. The stator 51 includes blocks 52-54, piezoelectric elements 55, 56, and electrode plates 57, 58. All the components of the stator 51 and the rotor 50 are connected by a through-bolt 59 and a nut 60, so that the rotor 50 slidably rotate in contact with the stator 51. One of the blocks 53 has slanted slits 53a formed thereon. When a high frequency voltage is imposed on the piezoelectric elements 55, 56, the piezoelectric elements generate a longitudinal vibration. A part of the longitudinal vibration is converted into a twisting vibration by slits 53a. The longitudinal and twisting vibrations are combined into a compound vibration which appears on a top surface of the stator 51. The rotor 50 slidably contacting the top surface of the rotor is rotated by the compound vibration. However, an efficiency in converting the longitudinal vibration into the twisting vibration by the slits is not sufficiently high in the conventional motor, which results in a low efficiency of the motor.
Also, another type of the ultrasonic motor which includes a piezoelectric element for generating a twisting vibration in place of the block 53 having the slits 53a has been known hitherto. The piezoelectric element for generating the twisting vibration is composed of plural pie-shaped elements, each element being polarized in a different direction from a neighboring element. The rotor is rotated by a compound vibration generated by both piezoelectric elements for the longitudinal vibration and the twisting vibration. Since the piezoelectric element for generating the twisting vibration is additionally required in this structure, and both piezoelectric elements have to be separately driven by high frequency voltages, the ultrasonic motor becomes complex and expensive.